Wireless optical communication system with adaptive data rates and/or adaptive levels of optical power

ABSTRACT

The wireless optical (in particular infrared) communication system with at least one transmitter (75) and one receiver (76) comprises control means (77, 78), which dynamically adapt the data rate and/or the optical power of the transmitter in dependence of signal-to-noise ratio of the receiver. Due to this adjustment, optimized system performance is maintained even under the influence of ambient light which statistically changes the signal-to-noise ratio of the receiver. The best compromise between data rate, bit error rate and transmission range is dynamically determined. The control function is distributed between transmitting and receiving system unit. The control information is communicated via wireless optical communication.

TECHNICAL FIELD

The present invention relates to a wireless optical communication systemfor data transmission.

BACKGROUND OF THE INVENTION

With the rapidly increasing number of workstations and personalcomputers (e.g. desktop or handheld ones) in all areas of business,administration, fabrication etc., there is also an increasing demand forflexible and simple interconnection of these systems. There is a similarneed as far as the hook-up and interconnection of peripheral devices,such as keyboards, computer mice, printers, plotters, scanners, displaysetc., is concerned. The use of electrical interconnects becomes aproblem with increasing number of systems communicating with each other,and in many cases in which the location of systems, or the configurationof subsystems, must be changed frequently. It is therefore desirable togain flexibility and thus to eliminate electrical interconnects for suchsystems and to use wireless communication instead.

The use of optical signals for wireless transfer of digital data betweensystems and devices has received increased interest during recent yearsand has lead to applications in commercial products. One example is theoptical remote control of electronic instruments. In the Patent Abstractof Japan of the Japanese Patent JP-A 5145975, No. 535 (E-1439), aconventional remote control system is disclosed. This remote controldetermines the amount of ambient light and sends out stronger signals ifthe ambient light level is high. If the ambient light level is low, ittransmits signals with reduced power. Another example is thecommunication between information systems in an office environment.Digital data to be transferred between a transmitting system and areceiving system are transformed to modulated optical signals which areradiated from a light source--in particular an infrared (IR) one--at thelocation of the transmitting system and are received and converted toelectrical signals and then to digital data by the receiving system. Inthe Patent Abstract of Japan of the Japanese Patent JP-A 4256234, No. 35(E-1310), an optical transmission system is disclosed. It comprises areceiver and transmitter being connected via feedback means. Thesefeedback means are employed to control the level of input currentdriving the light sources on the transmitter side. I.e., the systemdisclosed is a communication system where the output power of thetransmitter is controlled and adjusted to ensure that the burden of thelight sources is reduced. The optical signals might directly propagateto the optical receiver of the receiving system or they might indirectlyreach the receivers after changes of the direction of propagation due toprocesses like reflections or scattering at surfaces. Today, the formercase is realized in docking stations for portable computers where thedata transfer takes place between an optical transmitter and a receiverwhich are close together at a distance on the scale of cm and properlyaligned. The latter case is typical for applications in an officeenvironment in which undisturbed direct transmission of optical signalsbetween transmitters and receivers several meters away from each otheris unpractical or even impossible due to unavoidable perturbations ofthe direct path. One known approach to achieve a high degree offlexibility is to radiate optical signals from the transmitting systemto the ceiling of an office where they are reflected or diffuselyscattered. Thus, the radiation is distributed over a certain zone in thesurroundings of the transmitter. The distribution of the light signalsspreading from the ceiling depends on many details which arecharacteristic for the particular environment under consideration.However, essential in this context is mainly that the transmissionrange, i.e. the distance between transmitting system and receivingsystem, is limited to some final value, hereafter called thetransmission range, since the energy flux of the transmitted radiationdecreases with increasing distance of propagation and the receiversensitivity is limited due to a final signal-to-noise ratio. Typicalknown systems, operating at levels of optical power which are limited bythe performance of the light sources and safety requirements for lightexposure, have demonstrated transmission ranges of several meters fordata rates of 1 Mbps.

The latter example illustrates basic features of wireless opticalcommunication and indicates fields of applications where it is favorablyapplied in contrast to another competitive method of wirelesscommunication, the radio frequency (RF) transmission. Wireless opticalcommunication allows data transmission which is short range, whereas RFtransmission is potentially long range. Furthermore optical wirelesscommunication in an office environment is localized since typicalboundaries of an office such as walls and ceilings are not transparentfor light but for RF waves. That is why possible interferences betweendifferent communication systems are easier to control and a simpler wayfor achieving data security is possible for a wireless communicationsystem which is based on optical radiation rather than RF transmission.RF transmission is even restricted by communications regulations andlicenses whereas optical wireless communication systems are not.

Crucial parameters of a wireless optical communication system are theachievable data rate and the distance between the systems exchangingdata. In an office environment, it can be necessary to communicate dataover distances exceeding the transmission range of a single opticaltransmitter. However, the transmission range of a single opticaltransmitter can be extended within the concept of wirelesscommunication, for example by introducing optical repeaters. One exampleof such an extended system has been proposed in U.S. Pat. No. 4,402,090entitled "Communication System in which Data are Transferred BetweenTerminal Stations and Satellite Stations by Infrared Systems". In thispatent, a system is described which provides a plurality of satellitestations, i. e. stations usually fixed at the ceiling of a large room.Terminals can optically interact with satellites within theirtransmission range, and data can be distributed via intersatellitecommunication thus enabling the distribution of data over distancesbeyond the transmission range of a single transmitter.

When designing a wireless optical communication system, one has to beaware of unavoidable ambient light, such as daylight or light fromlamps, which always reaches the optical detectors, unless the system isrestricted for the use in a completely dark environment. Unavoidableambient light can lead to time-dependend signals, for example AC signalsfrom lamps, and is an important, in many practical cases the dominantsource of noise in the optical receiver. Thus, ambient light influencesthe signal-to-noise ratio of the receiver and, therefore, affects thetransmission range. The appearance of unavoidable light is mostlystatistical and often difficult to control and its intensity candrastically change, as it is apparent for sunlight or lamps beingswitched on/off. A further realistic effect which statistically affectsthe signal-to-noise ratio and thus the transmission range is theoccurrence of optical path obstructions influencing the receiver signal.In an office environment for example, moving users can change thestrength of the transmitted signals and the influence of unavoidableambient light as well.

In present wireless communication systems, first obvious attempts havebeen made to handle the ambient-light problem. Usually, low frequency(≦500 KHz) AC signals, which can be attributed to common roomillumination, are suppressed with electrical filters after theconversion of light to electrical signals. Optical filters are used torestrict the spectrum of undesired ambient light. However, a significantportion of daylight is spectrally in the same range as the opticalradiation of the light sources appropriate for wireless communicationsystems.

Present optical wireless communications systems which are designed forapplications in the presence of ambient light work with fixed data ratesand and optimization schemes. Today's systems, which operate at a fixedtransmission rate, offer the desired degree of data security only at theexpense of a reduction of the transmission range which corresponds tosecurity margins taking the influence of ambient light into account. Fortoday's systems, these security margins must be determined intrial-and-error experiments, individually for each particularconfiguration in each particular environment. Systems offering automaticcontrol and optimization of performance in the presence of ambient lightare not known.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a wireless opticalcommunication system which comprises at least one transmitting and onereceiving unit for optical signals and is suited for operation under thecondition that the optical receiver is exposed to unavoidable ambientlight, which deteriorates the receiver sensitivity. It is assumed thatthe exposure might statistically change with time.

It is another object of this invention to provide a method and anapparatus for optimizing the system performance under consideration ofdynamically changing exposures to ambient light.

The invention as claimed is intended to meet these objectives. Itprovides a method and an apparatus for improved wireless opticalcommunication. The improvement is achieved by introducing the opticalpower of the transmitting unit and/or the data rate as adaptableparameters, thus offering a useful extra degree of freedom and moreflexibility in the design of wireless optical communication systems.Furthermore, the optical power of a transmitting system and the datarate are parameters which can be set under automatic control. Suchcontrol can be achieved with many different means. A few examples ofsuch control means are cited in claims 1 to 12 and in the description ofthe invention. In addition, said parameters can be fixed values ofoptical power. In the Patent Abstract of Japan of the Japanese PatentJP-A 2042833, No. 200 (E-0920), an optical communication system isdisclosed. This system is characterized in that its transmitter portionemits light at high output power if frequent bit errors are detected. Ifthere are no bit errors or if the number of bit errors is low, theoutput power of the transmitter portion is reduced such that the energyconsumption is minimized. No case study is known which gives an analysisof how the trade-off between data rate and distance between thetransmitting and the receiving part of the system is influenced byambient light in a variety of situations representative for an officeenvironment. Since these trade-offs have not yet been studied for suchsystems, the benefit of control and optimization schemes which allow thedynamic optimization of wireless optical communication systems exposedto changing levels of ambient light with respect to transmission rate,transmission range and transmission security (bit error rate) has notbeen recognized. Therefore, no attempt has been made to introduce suchcontrol and optimization schemes. Today's systems, which operate at afixed transmission rate, offer the desired degree of data security onlyat the expense of a reduction of the transmission range whichcorresponds to security margins taking the influence of ambient lightinto account. For today's systems these security margins must bedetermined in trial-and-error experiments individually for eachparticular configuration in each particular environment. Systemsoffering automatic control and optimization of performance in thepresence of ambient light are not known.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a wireless opticalcommunication system which comprises at least one transmitting and onereceiving unit for optical signals and is suited for operation under thecondition that the optical receiver is exposed to unavoidable ambientlight, which deteriorates the receiver sensitivity. It is assumed thatthe exposure might statistically change with time.

It is another object of this invention to provide a method and anapparatus for optimizing the system performance under consideration ofdynamically changing exposures to ambient light.

The invention as claimed is intended to meet these objectives. Itprovides a method and an apparatus for improved wireless opticalcommunication. The improvement is achieved by introducing the opticalpower of the transmitting unit and/or the data rate as adaptableparameters, thus offering a useful extra degree of freedom and moreflexibility in the design of wireless optical communication systems.Furthermore, the optical power of a transmitting system and the datarate are parameters which can be set under automatic control. Suchcontrol can be achieved with many different means. In addition, saidparameters can be adapted automatically if required. This application isadequate for systems which are exposed to fluctuating ambient light. Forexample, taking the bit error rate as the main criterion, the data ratecan always be dynamically adapted to its momentary upper limit dependingon the exposure to ambient light.

In conclusion, this invention provides a method and an apparatus forimproved wireless optical communication. The improvement is achieved byintroducing automatic control means for the optical power of atransmitting unit and/or the data rate.

Advantages achievable with this invention are:

enhanced flexibility in system design;

simplification of integration of systems operating with different datarates;

dynamic performance optimization;

controlled bit error rates and thus data security even for adverseexposure to ambient light.

DESCRIPTION OF THE DRAWINGS AND NOTATIONS USED

The invention is described in detail below with reference to thefollowing drawings:

FIG. 1A shows a wireless IR link between a computer and a keyboard.

FIG. 1B shows a wireless IR network, sometimes called LAN on a table,interconnecting different computers and terminals as well as peripheraldevices (e.g. a printer).

FIG. 1C shows a wireless IR network with ring topology, called intraOffice LAN, interconnecting different computers and a mainframe.

FIG. 1D shows part of a wireless IR network with a repeater situated atthe ceiling, called Intra Office LAN with repeater, usually employed inopen area offices, conference rooms, or factory halls.

FIG. 2 shows three configurations of a transmitter/receiver pairconsidered as model systems for wireless optical intraofficecommunication.

FIG. 3 illustrates the received optical power plotted against thedistance S between a transmitter and receiver, for the three differenttransmitter/receiver configurations illustrated in FIG. 2.

FIG. 4 illustrates some examples of the bit error probability P_(e)versus distance S between receiver and transmitter.

FIG. 5 illustrates estimated relative data throughput T_(o) versusdistance S, for a vertical transmitter/receiver configuration, usingdifferent improvement schemes.

FIG. 6 illustrates the attainable transmission ranges for four differentdata rates (0.01 Mbps-10 Mbps).

FIGS. 7A-7B depict block diagrams for different architectures ofwireless optical communication systems comprising a transmitter/receiverpair and control means for optical power and/or data rate.

FIG. 8 shows an implementation of an optical receiver suited for theadaptation of the data rate. As an example a special design for signalswhich are encoded by pulse position modulation (PPM) is shown.

DETAILED DESCRIPTION

In general, a system for wireless optical communication comprises atleast one unit serving as transmitter and a second one serving asreceiver, the transmitter comprising a light source, such as a lightemitting diode (LED) or a laser diode, and the receiver comprising aphotodiode. The word unit is hereinafter used as a synonym for all kindsof computers, terminals, repeaters, peripheral devices etc., which mightcommunicate with each other, either unidirectional or bidirectional.Normally, infrared (IR) light is used for wireless optical communicationalthough the results presented in the following are not restricted to aspecific range of the light spectrum.

FIG. 1 shows four examples for applications of wireless opticalcommunication in an office environment, one basic transmitter/receiverconfiguration for direct IR communication and three configurations forindirect IR communication.

Direct transmitter/receiver coupling is well suited for applicationswhere only two, or just a few, units use the same IR channel. An exampleis illustrated in FIG. 1A. In this Figure, a first unit, for example akeyboard 21, is coupled to a second unit, a computer 20. This kind ofwireless IR link might be unidirectional and the maximum distance isusually less than 1 meter. The direct line-of-sight path between thesetwo units has to be obstruction-free to facilitate reliable operation.

A wireless IR network, sometimes called LAN on a table, is illustratedin FIG. 1B. As shown in this Figure, three different units are linked toa fourth one. In the present example, two computers 23 and 25 and aterminal 24 are linked to a printer 22. Direct as well as indirectconfigurations are suitable for these kind of applications.

In FIG. 1C, a wireless IR network with ring topology, called IntraOffice LAN is shown. This IR network interconnects three computers 27with a mainframe machine 26. Usually indirect configurations are bettersuited for Intra Office IR networks.

Another exemplary IR network configuration is shown in FIG. 1D. A firstunit, e.g. a repeater 28, is situated at the ceiling in order to be ableto communicate with remote units. In the present example the remoteunits are computers 29. Such a configuration is usually called IntraOffice LAN with repeater, and might be employed in open area offices,conference rooms, and factory halls.

In the following an evaluation of the performance limits of wirelessoptical communication systems is presented. For the sake of simplicitythree different configurations of a single transmitter/receiver pair areconsidered, a vertical transmitter/receiver configuration, a tiltedtransmitter/receiver configuration and a spotlight transmitter/receiverconfiguration (see FIG. 2). As the following analyses show, these threeexamples have similar performance characteristics which differ onlyslightly. Thus these examples are considered as representative models.As measures of their performance, the data rate, the bit error rate andthe distance between transmitter and receiver are taken. In a firststep, the trade-offs between these parameters are derived from analysesof the signal-to-noise ratio and a calculation of the probability forthe occurrence of a bit error (bit error rate). In a second step, theinfluence of ambient light is included. On this basis optimizationschemes are discussed.

The formulae given in the following sections provide a reasonableapproximation of the power received at the photodiode as a function ofthe distance between the transmitter 10 and the receiver 11. It isassumed that the transmitter emits a narrow parallel beam which isreflected at the ceiling or a similar surface as a diffuse (Lambertian)point source. The signal power incident on the photodiode is then givenas the radiation contained in the solid angle bounded by the projectedphotodiode area. It is assumed that the path of the propagating light isnot obstructed. The following parameters are used:

    ______________________________________                                        P.sub.s = 1 Watt                                                                           average optical power of the transmitter                         A.sub.r = 1 cm.sup.2                                                                       photodiode area                                                  H = 1.8 m    height of ceiling above desk top                                 ρ = 0.7  reflection coefficient of ceiling                                S = 0-20 m   distance between transmitter and receiver                        ______________________________________                                    

Vertical Transmitter/receiver Configuration

This first indirect configuration, illustrated in FIG. 2A, ischaracterized in that the LED of the transmitter and the photodiode ofthe receiver point upward and normal to the ceiling of a room. Thisconfiguration does not need alignment of the transmitter and receiver,but produces a 4th-power signal attenuation with distance S, S being thedistance between the transmitter and the receiver. The received signalpower is approximately given by ##EQU1## It has been experimentallyfound that formula (1) underestimates the power levels with increasingdistance S. An approximate correction can be made by multiplyingequation (1) with a correction factor. This correction is necessarysince multiple reflections have not been taken into account.

Tilted Transmitter/receiver Configuration

This configuration (see FIG. 2B) requires that the LEDs of alltransmitters and the photodiodes of the receivers are approximatelydirected towards the center of the ceiling of the room. In practice, itsuffices that remote transmitters and receivers located at the peripheryof the transmission range are tilted by approximately 45° and face theoffice interior, whereas other transmitters and receivers located at thecenter are pointing upward. The advantages of the tilted configurationare:

1. The signal power is spread more uniformly thus allowing a greatertransmission range.

2. In most cases, direct exposure to sunlight or desk lamps can beavoided.

3. Transmitters and/or receivers located at the periphery can in manycases benefit from a direct line-of-sight path thus increasing the powerefficiency.

However, this approach requires a flexible integration of thetransmitter and receiver into the unit's housing. In case of a tiltedconfiguration the received signal power is approximated by theexpression ##EQU2##

Spotlight Transmitter/receiver Configuration

This particular configuration is characterized in that, in addition tothe common alignment of all transmitters and receivers, a colliminatednarrow LED beam is required, allowing the reflected spot to appear atthe intersection of the LED axis with the ceiling. The reflected diffusepoint source therefore appears halfway between the most distanttransmitter/receiver pair, resulting in the smallest propagation loss.The corresponding expression for the received signal power P_(r) is then##EQU3## Since LEDs with small beam angles are neither easily producednor commercially available, other light sources with small half-powerangles are required. A colliminated laser source, for example, couldsatisfy the above conditions. The resulting narrow field-of-view wouldalso allow the use of large aperture lenses with considerable opticalgain, as well as narrow optical bandpass filters to suppress theundesired ambient light outside the spectrum of the optical signalsource. It is a disadvantage of this concept that the complicatedalignment procedure is not suited for user-friendly mobile applications.Note that, when herein referring to optical signal sources, alldifferent kinds of diodes, including the conventional LEDs as well aslaser diodes, are meant.

In FIG. 3, the received optical power P_(r) is plotted against distanceS for the three basic indirect transmitter/receiver configurationsaddressed above. The diagram in FIG. 3 is based on the assumption thatthe source power P_(s) =1 W and the photodiode area A_(r) =1 cm². Inaddition, the transmitter is assumed to be located at the position S=0,whereas the receiver is moved a distance S.

From Equations (1)-(3) the receiver signals can be obtained for eachconfiguration. In the following these results are related to thereceiver noise and subsequently converted to the bit error probabilityP_(e) as a function of distance S. At this point the influence of theambient light environment can be taken into account as contribution tothe shot noise of the receiver. A simple model is assumed that estimatesthe bit error probability P_(e). as a function of the distance S and theshot noise generated by different ambient light environments. Thefollowing parameters are used in addition to the Boltzmann constant k,the absolute temperature T, and the electron charge e:

    ______________________________________                                        η = 0.5 A/W  photodiode efficiency                                        R.sub.1 = 1 kΩ                                                                           photodiode bias resistor                                     ______________________________________                                    

The mean square noise current is given by ##EQU4## where B is theelectrical bandwidth of the receiver, and I_(b) the photodiode biascurrent due to imperfect optical filtering of the ambient light. Thefirst noise term represents a thermal noise floor (preamplifier noiseassumed included) which is present at all times. Note that due to theassumed low 1 kΩ value (to prevent excessive photodiode bias voltages)the noise floor is rather high. In practice, lower noise levels can berealized, resulting in improved transmission distances for fluorescentenvironments. The shot noise term depends on the ambient light levelpassing an optical filter situated in front of the receiving photodiode.Different kinds of optical filters, if any, such as optical interferencefilters or absorption filters, might be used.

We assume the transmission of a binary data stream consisting of asequence of symbols, either "0" or "1", each symbol denoting one bit ofinformation, the "1" being represented by a single optical pulse ofduration T_(p) and the "0" being represented by the lack of a signalduring the time span T_(p). For this particular coding scheme, the timeper transmitted bit, T_(b), is equal to T_(p), and the data rate of thetransmission generally defined as bit rate R_(b) =1/T_(b), i. e. themomentary speed at which the bits of information are transmitted andrecognized as "0" or "1" by the receiver, is equal to R_(b) =1/T_(p).

In order to assure that the receiver transmits a single pulse withoutsignificant distortion but suppresses noise as good as possible, weassume the relation

    B≃1/T.sub.b =R.sub.b                         (5)

for the bandwidth B of the receiver. The mean signal current is relatedto the received signal power P_(r) through

    i.sub.s =P.sub.r η                                     (6)

and the signal-to-noise (S/N) ratio is defined by ##EQU5##

    α=i.sub.s /i.sub.n                                   (7)

The bit error probability for binary transmission and white Gaussiannoise is given by the error function which is herein approximated with##EQU6## to gain a simple analytical expression which, however,overestimates the bit error probability. In FIG. 4 some examples of thebit error rate P_(e) versus distance S are shown which illustrate theexistence of a well defined communication cutoff distance for a givenambient light environment. FIG. 4 holds for the data rate R_(b) =1 Mbps.As ambient light environment, exposure by full sunlight (full lines) andlight of fluorescent lamps (dashed-dotted lines) has been chosen.

The probability of at most m errors occurring in a data packetcontaining n bits (assuming independent bit errors) is given by thecumulative binominal distribution ##EQU7## To estimate the datathroughput, i. e. the average speed of the transmission of dataexcluding overhead such as address information, idle bits etc., weassume a "Stop and Wait Automatic Repeat Request (ARQ)" transmissionprocedure. With m=0 (zero errors occurring in the packet) the relativedata throughput, which is normalized with respect to the maximum datarate R_(max), a design parameter of the system, is given by ##EQU8## Wewish to analyze Equation (10) for the parameters

R_(max) =10 Mbps or 1 Mbps

R_(b) =10 Mbps, 1 Mbps, 0.1 Mbps, 0.01 Mbps

    ______________________________________                                        d = 1024  number of data bits per packet                                      n = 1064  number of total bits per packet, including addresses                          and CRC (cyclic redundancy check)                                   p = 16    number of preamble bits in a packet                                 i = 72    number of idle bit intervals between packets                        ______________________________________                                    

For this particular example, the maximum throughput (at R_(b) =R_(max))is 0.889 due to the assumed ratio of payload to overhead.

The estimated data throughputs T_(o) versus distance S for verticaltransmitter/receiver alignment using the following four known exemplaryimprovement schemes are illustrated in FIG. 5. As an example the datarate R_(b) =R_(max) =1 Mbps has been considered.

Optical Absorption Filter (Standard Version)

The transmission limit in direct sunlight is given by the filled area inFIG. 5 and amounts to only 2.5 to 3 meters. A similar limit has beenverified with measurements of conventional IR systems. The range in afluorescent light environment is indicated by the thin solid line (≃7m).

Optical Interference (IF) Filter with Optical Bandwidth Corresponding tothe Width of a Typical LED Emission Spectrum (δλ≃50 nm)

The range improvement is shown with the heavy and thin dashed lines fordirect sunlight and fluorescent light, respectively. The improvement isabout 0.5 meters for direct sunlight. Since fluorescent light containsonly little IR-radiation, nearly no improvement can be gained in thiscase.

Error Correction Encoding

The use of an error correction code allows a limited number of corruptedbits to be restored which is equivalent to allowing a smaller signallevel for a given noise level (coding gain). This gain might be used toimprove the transmission range somewhat. For a commercially availableReed-Solomon Encoder/Decoder chip set a coding gain of 3 dB was assumed.The combined effect of the IF-filter and the coding gain is shown withthe dashed-dotted lines providing a range improvement of ≃1 meter.

Variable Packet Sizes

Transmitting very short packets improves the probability of receivinguncorrupted messages for a given bit error probability. However, asfound by carrying out different measurements, the range improvement isnegligible.

In FIG. 6 the attainable transmission ranges for a tiltedtransmitter/receiver configuration are estimated for four different datarates (0.01 Mbps-10 Mbps). With 0.1 Mbps a transmission range of up to10 meters can be achieved with the transmitters and receivers exposed todirect sunlight, as illustrated in FIG. 6. The open and full circles inFIG. 6 represent experimental values.

From FIG. 5 and FIG. 6, general design criteria for wireless opticalcommunication systems which operate with optimized performance in anambient light environment can be deduced. The transmission range of asystem working with 10 Mbps is limited to roughly 2 m if typical extremecases for exposures to ambient light are considered. On the other hand,perfect (error free) transmission over `long` distances (≃10 m) requiresan extremely low data rate (10 kbps). Therefore, practical applicationsof wireless optical communication systems are rather limited if they areoperated at a fixed data rate. Such systems are either fast andshort-range or slow and long-range. However, today's applicationsrequire more design flexibility. Unfortunately, the conventionalimprovement schemes mentioned above can only compensate a negligibleportion of the effect which can be attributed to ambient light.

In accordance with this invention, the desired gain in designflexibility can be achieved by using the data rate and the optical powerof the transmitter as adaptable parameters and introducing control meansfor their control. Automation of this control procedure allows fordynamic optimization in the sense that the best compromise between datarate and transmission range can always be found for a predefined biterror rate.

The control of the optical power and the data rate are related to thecontrol of the signal-to-noise ratio of the receiver. The optical powerof the transmitter influences the signal of the receiver. However, themaximum data rate corresponds to the smallest signal-to-noise ratiowhich is compatible with a predefined bit error rate and, therefore, tothe signal bandwidth of the receiver. Therefore, a method which changesthe data rate corresponds to a method which changes the suppression ofnoise with respect to the signal.

Methods influencing optical power and/or data rates are known. The powerof the light source of the transmitter can be influenced by the drivecurrent which can be automatically controlled by means which are stateof the art.

Alternatively, light modulators could be used. Examples for such devicesare electrooptic modulators, based on electroabsorption orelectrorefraction. From the signal-to-noise point of view, it isfavorable to operate the light source at the highest power level whichis limited by the device performance and safety requirements. The datarate is basically defined by the chosen coding scheme and the time perpulse T_(p). The control of the data rate has two aspects, namely, howto influence the data rate and how to communicate the information aboutthe proper data rate between transmitter and receiver, i.e. how tosynchronize transmitter and receiver. As far as methods affecting thedata rate are concerned, a change of T_(p) relates to a modification ofthe electrical bandwidth B of the receiver and thus to a change of thereceiver's noise. B can be controlled with an adjustable electricalfilter. Such devices are known. One example of how to influence the datarate via a particular coding scheme even for a constant time per bitT_(b) and a constant time per pulse T_(p) is the multiple transmissionof redundant information. In this case individual symbols of the code,each related to a time frame of a given duration T₂ and eachrepresenting a certain number of bits, are transmitted m times, m beingan integer. This multiple transmission reduces the data rate by 1/m, butenables the application of noise suppression procedures such as signalaveraging, leading to an improvement of the signal-to-noise ratio of thereceiver by roughly a factor 1/√m, even if the electrical bandwidth ofthe receiver is left unchanged. This example and additional concepts forthe adjustment of data rates are discussed below in the context with anembodiment in accordance with this invention. A realization of thetransmitter/receiver synchronization is also given there.

The block diagrams in FIG. 7 show how such control processes could beorganized in general. A control system might act as an independentsystem 72 which interacts with the transmitter 70 and the receiver 71for setting data rate and/or optical power (FIG. 7A). Input parametersfor the control system might be a measure for the signal-to-noise-ratioof the receiver 71 or signals from detectors which characterize theambient light. In accordance with this invention the information betweenthe control system 72 and transmitter 70 and receiver 71 could betransferred via wireless optical communication. In this case thereceiver must comprise an additional optical transmitter and thetransmitter has to comprise an additional optical receiver. Anotherrealization of the same inventive concept is the integration of thecontrol function (77, 78) into the transmitting unit and the receivingunit itself (FIG. 7B). The transmitting and the receiving unit canexchange all information about data rate and/or optical power in a handshake process. Again, wireless optical communication is an adequatemethod for this procedure in accordance with this invention.

In the following, a receiver which is in accordance with the presentinvention is described. The receiver is illustrated in FIG. 8. Anexample for the synchronization of transmitter and receiver is alsogiven below.

As data encoding scheme, Pulse Position Modulation (PPM) is assumed,i.e. the data stream is split up into a sequence of packets. Each packetdefines a sequence of time frames of duration T₂. By definition, n bitsare represented by m equivalent pulses each of them being related to oneof m subsequent time frames, having the duration T_(p) =T₂ /2^(n) andbeing identified by one of 2^(n) possible equidistant positions withineach time frame. This particular definition of PPM-encoding includes thepossibility of repeating the same information, encoded by the positionof a single pulse with respect to one time frame, m times. Thus, in thegeneral case m≧1 the data rate, i. e. the number of transmitted bits pertime of transmission, is given by ##EQU9## A reasonable compromisebetween the requirement of transmitting pulses without significantdistortion and suppressing noise as much as possible is found forsetting the receiver bandwidth B≃1/T_(p).

In this type of encoding, the possibilities for changing the data rateare at least threefold. On the one hand, the number n of bits per timeframe and thus T₂ can be changed in combination with the optical outputpower of the transmitter. However, in many cases the application of thisapproach is limited due to power efficiency considerations. Often it isdesired to achieve the highest signal possible. In this case, it isuseful to operate the light source of the transmitting unit at thehighest power levels which are compatible with safety restrictions andthe limits of the device performance. Usually upper limits for theaverage and the peak of the optical power must be defined. Therefore,also the number n of bits related to a single time frame has an upperlimit. Performance data of typical known LEDs suggest to choose n=4 andT_(p) ≃250 ns for a transmission with the data rate 1 Mbps. A secondapproach is affecting the noise level by changing the receiver'sbandwidth B in combination with the pulse duration T_(p) according tothe relation given above. Third, if B and T_(p) are fixed, thetransmission of each time frame can be repeated m times within a singlepacket, thus reducing the data rate by 1/m with respect to the case m=1.Digital signal processing of the received m equal frames, as describedlater, will decrease the bit error rate.

The receiver illustrated in FIG. 8 comprises an opto-electronic receiverwith photodiode 34. The received optical signal is converted to anelectrical signal which is fed to the amplifier 35. An optional gaincontrol circuit 45 (AGC) might be employed in order to keep theamplitudes at the output of the amplifier 35 constant. A bandpass filter46 provides a bandpass-filtered signal (with bandwidth˜B) which is fedto a slicer 47. Means 48 for baseline restoration are provided toextract the baseline signal from the signal at the output of amplifier35. This baseline signal forwarded from the means 48 for baselinerestoration to said slicer 47 is not constant due to ac coupling. Harddecisions on detected pulses (true pulses or noise) are clocked into ashift register 50. The shift register 50 has 2^(n) cells in order tocontain one frame length. The clock signal φ_(P) for triggering saidregister 50 is generated using means 49 for preamble processing. Forenabling transmitter/receiver synchronization and proper processing ofreceived data, a sequence of preamble bits, which carries signals forthe synchronization of the system clock and for the synchronization ofthe time frame T₂ and delivers encoded information about the data rate(i.e. n and m), is transmitted at the beginning of each data packet. Thepreamble processor 49 provides signals for clock extraction 59.1, framesynchronization 59.2, data rate detection 59.2, and carrier sensing59.3. The means 49 for preamble processing are assumed to deliver clockpulses φ_(P) starting at the beginning of the first frame of thepreamble.

The shift register 50 provides 2^(n) output signals forwarded tocounters (flip flops) 54.1 through 54.x. With no errors, only onecounter will contain the detected pulse in the correct position. Witherrors, several counters may contain a "pulse". At the end of eachframe, the output of the shift register 50 are clocked into saidcounters 54.1-54.x., triggered by a counter clock φ_(F) obtained from afirst divider 51. This first divider 51 divides the clock pulse φ_(P) by2^(n).

In case of transmission at highest speed, i.e. with m=1, all frames areonly transmitted once. The contents of the counters 54.1-54.x are thentransferred to means 55 for bit position estimation with a clock φ_(MF).The bit position estimator 55 makes an attempt to relate a detectedpulse to its position with respect to its corresponding time frame T₂.The clock φ_(MF) is equal to φ_(F) except a phase shift. After havingthe contents of the counter transferred to the bit position estimator55, the counters are reset by a signal provided at an output of a seconddivider 52. If no error occurred, only one counter contains the pulsecount "1" and all others "0". In other words, the bit position estimatordelivers a measure of the signal-to-noise ratio of the receiver and,equivalently, the bit error rate. From the results of the bit positionestimation, the transmitted data are extracted by the decoder 56, andserialized by means 57 which receives trigger signals from means 53. Theinterface logic 58 makes the received data available for sub sequentdata processing.

In case of repeated transmission, e.g. with m=10, 100, or 1000, witheach clock φ_(F) the counters are incremented by the contents of theshift register 50. Here the clock signal provided by said second divider52 is φ_(MF) =φ_(F) /m, i.e. this divider divides the clock signal by m.After m frames, the contents of the counters are transferred to saidmeans 55 for bit position estimation. Then, the counters are reset by atrigger signal 59.4 generated by divider 52. In this way, the countersperform signal averaging of 2^(n) samples of the optical signal receivedduring one time frame T₂. Thus, they deliver a sampled signal whosesignal-to-noise ratio is improved by a factor 1/√m.

For adapting the data rate by electrical filtering, the adjustment ofthe width of the bandpass filter of the receiver is required. For thispurpose, adjustable analog or digital filters are needed. The pulselengths are much longer at low data rates such that the power of thetransmitter's light source (e.g. a LED) must be reduced to preventoverheating. It is a disadvantage of this method that data rates belowabout 500 kHz are not possible. This part of the frequency spectrum mustbe completely suppressed to eliminate the dominant noise contributiondue to fluorescent lamps.

According to this invention, the receiver described above can be used ina wireless optical communication system with adaptive data rates in thefollowing way. PPM encoding is chosen. It is assumed that the parametersm and n, i.e the number of repetitions of each time frame and the numberof bits per time frame, respectively, are taken as control parametersfor the data rate in addition to the optical power of the transmitter.As mentioned above, all information about clock and framesynchronization and the data rate are contained in the sequence ofpreamble bits of each data packet. Furthermore, synchronization of clockand frame and proper data processing in accordance with predefinedvalues for m and n is controlled by the preamble processor 49. Startingfrom these prescriptions, a control means in accordance with thisinvention is described. As an example, the system architecture shown inFIG. 7B is used, i. e. the control function is distributed between thetransmitter and the receiver. For the exchange of control data, wirelessoptical communication is used, i. e. the transmitting unit of the systemcomprises a receiver as shown in FIG. 8, and the receiving unit of thesystem comprises an optical transmitter which might be of the same typeas the one in the transmitting unit of system. Since all informationrelated to the synchronization of the transmitting and receiving systemunits is included in the communication protocol, namely the preamble bitsequence, only a reasonable sequence of control steps needs to bedefined for establishing a synchronization and optimization procedure onthe basis of a handshake mechanism, which can be organized byindependent processors in the transmitting and the receiving systemsunits.

One possible handshake procedure works as follows. At the beginning of acommunication process, predefined values for the controlparameters--namely m, n and the optical power of the transmitters--arechosen, m and n being known to the control processors of thetransmitting system unit and the receiving system unit as well. It isreasonable to start a transmission of test signals at a low default datarate in order to realize signals with a reasonable signal-to-noise ratiowhich allows for unmistakable optimizing steps. As test signals, thepreamble bit pattern of the first data packet to be transmitted could beused. As a result of this first attempt to start a communicationprocess, the receiver, especially its bit position estimator and itsdecoder, delivers a measure of the actual signal-to-noise ratio and thebit error rate. Taking these data, the control processor of thereceiving system unit determines whether these data are betweenpredefined limits and whether there is room for improvement for the datarate and/or the optical output power of the transmitter. The rulesaccording to which a new set of the adaptable control parameters istaken, can be given by mathematical relations which might be determinedexperimentally or by means of modelling calculations. In a reverseprocess, the control processor of the transmitting unit expectsinformation about possible improvements being transmitted from thereceiving unit, and reacts with the command for the continuation of thesynchronization process using a new set of values for the controlparameters. If no response from the receiver appears, the transmittingunit might make an attempt to establish communication by subsequentlydecreasing the transmission rate and thus improving the signal-to-noiseratio. This procedure stops either after having determined an optimizedset of control parameters or after having found that communication isimpossible within the degrees of freedom of the system. If thecommunication is established once, the receiving unit can send a requestfor changing the control parameters whenever the signal-to-noise ratiochanges, and the transmitting unit reacts accordingly.

A further degree of freedom for changing the data rate can be introducedby allowing for switching between different coding schemes. Startingfrom the PPM-based system described above and assuming a given pulseduration T_(p) and time frames with given duration T₂, the data rate canbe increased by adding additional pulses to each time frame, thusincreasing the number of bits which are related to a single time framewith T₂ /T_(p) possible pulse positions. Due to limitations of theaverage output power of the transmitting unit, the adding of additionalpulses might require a reduction of the peak power. In order to realizethis approach the PPM-based system described above must be modified.First, the preamble bit pattern of each packet must include informationabout the coding scheme used. Second, the preamble processor 49 must bemodified for being enabled to handle the preamble. Furthermore, theinformation about the proper coding scheme must be forwarded to thedecoder 56 whose function must depend on the coding scheme. The sameholds for the bit position estimator if its content is used for theestimation of the signal-to-noise ratio and/or the bit error rate.

In conclusion, based on analyses of the data throughput, a method and anapparatus for wireless optical communication with adaptive data ratesand/or levels of optical output power is proposed which allows foroptimizing the data throughput for a particular distance and ambientlight environment. In accordance with the present invention full networkconnectivity within a prescribed range (e.g. 10×10 m) can be maintainedat the expense of (often temporarily) reduced throughput. A low datarate, e.g. 0.01 Mbps, may still be sufficient for connecting peripheraldevices such as printers 22, modems, keyboards 21 etc. to remote units20, 23, 24, 25, as illustrated in FIGS. 1A and 1B. In addition,obstructions of the propagation path (for instance by a person obscuringthe photodiode of a receiver) can be taken into account by transientresorting to a lower data rate if necessary. Experiments have shown thata person standing 30 cm away from a receiver can cause a 5 dB to 7 dBoptical power drop (tilted transmitter/receiver configuration located atdesktop level in opposite corners of a 10 m×10 m room). While fullnetwork connectivity is maintained due to the present invention even in`normal` adverse conditions, the user may only notice a gracefuldegradation in throughput instead of an abrupt communication cutoff.

When employing the present invention in an IR network with repeaterwhich retransmits correctly received data packets, as illustrated inFIG. 1D, the overall network throughput can be increased. Alternatively,one or several participating units (stations) may be configured toretransmit packets not addressed to themselves. As an example (see FIG.5), a packet transmitted from a transceiver of a first unit at 0.1 Mbpscan reach the transceiver of another unit--exposed to directsunlight--and being separated some 7-10 meters from the first unit,resulting in a throughput of ≃1%. With a repeater station inbetween, thefull 10 Mbps rate can be maintained resulting in a throughput of ≃50%(packet transmitted twice). The repeater concept is also suited toincrease the overall network range which is important in large offices,for example.

I claim:
 1. A wireless optical communication system for datatransmission with at least one transmitting unit for radiating modulatedoptical signals and at least one receiving unit for receiving saidoptical signals, characterized by control means comprising:means todetermine an optimized set of control parameters based on informationreflecting the actual bit error rate provided by said receiving unit,means for facilitating the optical wireless exchange of control databetween said receiving unit and transmitting unit, and means for dynamicadaptation of the data rate of said data transmission to ensure thatsaid actual bit error rate does not exceed a predefined upper limit. 2.The communication system of claim 1, wherein the control means comprisesat least one processor which receives information about said bit errorrate of the data transmission from the receiving unit.
 3. Thecommunication system of claim 2, wherein the receiving unit comprises atransmitter for optical signals and the transmitting unit comprises areceiver for optical signals for exchanging said control data betweensaid receiving unit and transmitting unit via wireless opticalcommunication.
 4. The communication system of claim 1, wherein thecontrol means comprise at least two processors, one being part of thetransmitting unit and one being part of the receiving unit, bothcommunicating with each other for setting the data rate of the datatransmission in an interactive process based on said control dataexchanged.
 5. The communication system of claim 4, wherein theprocessors communicate via bidirectional wireless optical communication.6. The communication system of claim 1, wherein the control meansincludes at least one optical detector, which is used for determiningthe intensity of ambient light.
 7. A wireless optical communicationsystem for data transmission comprising:at least one transmitting unitfor radiating modulated optical signals, at least one receiving unit forreceiving said optical signals, the recieving unit having a detector foroptical radiation which converts optical signals to electrical signals;an amplifier and a bandpass filter for said electrical signals; a signalaverager which periodically samples incoming electrical signals during atime frame of a predefined duration T₁, and superposes said sampledsignals of subsequent time frames m times, where m is a predefinedinteger; and a decoding system for the extraction of the data from thesignals after being processed by the signal averager, and a controlmeans having a means to determine an optimized set of control parametersbased on information reflecting the actual bit error rate provided bysaid receiving unit, means for facilitating the optical wirelessexchange of control data between said receiving unit and transmittingunit, and a means for dynamic adaptation of the data rate of said datatransmission to ensure that said actual bit error rate does not exceed apredefined upper limit, the control means at least two processors, onebeing part of the transmitting unit and one being part of the receivingunit, both communicating with each other for setting the data rate ofthe data transmission in an interactive process based on said controldata exchanged wherein the processors communicate via bidirectionalwireless optical communication.
 8. The communication system of claim 7,wherein the data rate is adapted by modifying the time per pulse T_(p)in combination with the corresponding modification of the electricalbandwidth B of the receiver in accordance with B≃1/T_(P).
 9. Thecommunication system of claim 8, wherein the data are split into subsetswhich are transmitted with k subsequent repetitions, where k is apredefined integer≧1 and each subset has a predefined duration T₂. 10.The communication system of claim 9, whereinthe data rate is adapted bychanging the number k of said repetitions according to predefined rules;and the signal averager and the decoding system are synchronized to thetransmission of the packets, T₁ ≧T₂ and k=m.
 11. The communicationsystem of claim 10, whereineach subset carries n bits which are coded bypulse position modulation (PPM) within the duration T₂, the receivershave means for decoding PPM-coded data.
 12. The communication system ofclaim 11, wherein the data rate is adapted by changing n in combinationwith an adjustment of the optical power of the transmitting unit.
 13. Amethod for wireless optical data communication between at least onetransmitting unit and at least one receiving unit, comprising the stepsof,radiating optical signals from said transmitting unit; detecting saidoptical signals by the receiving unit; determining an optimized set ofcontrol parameters based on information reflecting the actual bit errorrate of the transmission, optically exchanging control data between saidreceiving unit and transmitting unit, and dynamically adjusting the datarate of the transmission to ensure that said actual bit error rate doesnot exceed a predefined upper limit.
 14. A method for wireless opticaldata communication between at least one transmitting unit and at leastone receiving unit, comprising the steps of:radiating optical signalsfrom said transmitting unit; detecting said optical signals by thereceiving unit; determining an optimized set of control parameters basedon information reflecting the actual bit error rate of the transmission,optically exchanging control data between said receiving unit andtransmitting unit, and dynamically adjusting the data rate of thetransmission to ensure that said actual bit error rate does not exceed apredefined upper limit converting the detected optical signals toelectrical signal; amplifying and filtering said electrical signals;sampling said electrical signals during a time frame of predefinedduration T₁ ; averaging said sampled signals related to m subsequenttime frames, m being an integer, and decoding said electrical signals.15. The method of claim 14, wherein the step of adjusting the data ratecomprises the step of modifying the time per pulse T_(P) in combinationwith changing the bandwidth B of the receiver in accordance withB≃1/T_(P).
 16. The method of claim 15, wherein the data transmission isbased on the steps of splitting up the data in subsets of duration T₂and transmitting each subset with k subsequent repetitions, k being≧1.17. The method of claim 16, comprising the steps ofadapting the datarate by changing the number k of the repetitions according to predefinedrules, synchronizing the time frames for the sampling of signals withthe subsets, and averaging the sampled signals of m equivalent timeframes.
 18. The method of claim 17, wherein the data transmissionentails the coding of n bits, n being an integer, by pulse positionmodulation (PPM) within each subset.
 19. The method of claim 18,comprising the step of adapting the data rate by changing n incombination with an adjustment of the optical power of the transmittingunit.
 20. A receiving unit for use in a wireless optical communicationsystem comprising:a receiver for optical signals, means to determine theactual bit error rate, means facilitating the optical wireless exchangeof control data between said receiving unit and a remote transmittingunit, and means to interactively determine an optimized set of controlparameters taking into account information reflecting said actual biterror rate.
 21. A receiving unit for use in a wireless opticalcommunication system comprising:a receiver for optical signals; means todetermine the actual bit error rate; means facilitating the opticalwireless exchange of control data between said receiving unit and aremote transmitting unit; means to interactively determine an optimizedset of control parameters taking into account information reflectingsaid actual bit error rate; a detector for optical radiation whichconverts optical signals to electrical signals; an amplifier and abandpass filter for said electrical signals; a signal averager whichperiodically samples incoming electrical signals during a time frame ofa predefined duration T₁, and superposes said sampled signals ofsubsequent time frames m times, where m is a predefined integer; and adecoding system for the extraction of the data from the signals afterbeing processed by the signal averager.